Determination of activity concentration of 238U and 232Th series radionuclides in soil using a gamma-ray spectrometer in singles and coincidence modes

Design, construction and characterisation of a portable gamma-ray spectrometer for low-level natural occurring radioactive material ex-situ measurement

The activity concentrations in naturally occurring radioactive material (NORM) samples are conventionally measured using a gamma-ray spectrometer with a single detector (mostly HPGe or NaI:Tl) enclosed in a lead shield. In this work, a passive water shield to reduce background radiation reaching the detectors was designed using GEANT4-toolkit Monte Carlo simulations and then constructed. This measurement system is portable and cost-effective for ex-situ measurements. IAEA-375 soil and beach sand each placed in Marinelli beakers were measured using two LaBr₃:Ce detectors in singles and coin-cidence modes. A novel method of background reduction by using photon time-of-flight was employed together with the measurement of the two photons energies. These samples were also measured in singles mode using a NaI:Tl detector inside the constructed water shield and HPGe detector shielded with lead to compare and validate the results of LaBr₃:Ce detectors measurement. Both the simulated and measured results show that the water shield (500 mm thick) attenuates 2614.5 keV gamma rays by 90 %. The minimum detectable activity (MDA) measured for 24 h using the NaI:Tl detector without shield and inside the water shield are; 546 Bq kg^-1 and 146 Bq kg^-1 at 1460.8 keV (⁴⁰K), 194 Bq kg^-1 and 15 Bq kg^-1 at 1764.5 keV (²³⁸U series), and 131 Bq kg^-1 and 15 Bq kg^-1 at 911.2 keV (²³²Th series), respectively. The measured activity concentrations of ²³⁸U and ²³²Th series and ⁴⁰K radionuclides inside IAEA-375 soil agree with certified values to within measurement uncertainties. The measured activity of ¹³⁸La in the LaBr₃:Ce detector crystal is 263.8 ± 26.8 Bq kg^-1. The internal activity of the LaBr₃:Ce detector increases the MDA at 1460.8 keV and 2614.5 keV, which limits the measurement of primordial radionuclides with low activity concentration in singles mode. The activity concentrations of ²³⁸U and ²³²Th series radionuclides in beach sand were measured using NaI:Tl and LaBr₃:Ce detectors. The results are consistent with those from HPGe measurement to within uncertainties.